If more people understood animal hierarchy, there would be less aggression, less fighting, less blood spilt, fewer lives lost and certainly less dough spent on panel beating and repairs.
Trouble is, we humans often think we know better and interfere with the natural animal hierarchy which is written in stone and cannot be changed no matter how clever we think we are.  And yes, I have read all the important books and scientific papers on the subject of animal behaviour.

I’ve also listened to experts waxing lyrical on how the biggest, strongest, bravest, best connected, quickest or most virile animal wins the territory, claims the spoils and becomes King of the Castle. But the truth is, they are guessing or reinventing each other’s ideas because theorising about brain functions in order to arrive at the right answer can be unreliable according to my humble observations.

Hierarchy is determined by age

And in spite of neurotransmitters and autonomic ganglia which sound like car parts to me, all animals have a hierarchy or pecking order which cannot be denied. But here is a shock for the experts … dominance is determined by chronological age - animals can tell each others ages…. don’t know how but it’s true - an older animal, unless it has abdicated due to old age and self preservation, will never submit to a younger animal! But with intervention, a younger animal can be made to submit to an older …   and the fighting will cease forthwith.

Now this snippet of insight will help anyone with warring pets. Let’s take dogs for example. Your ‘new’ pup was probably overprotected and escaped obligatory pack initiation rights and the usual animal hierarchy. So Rambo grew up too big for his fake boots and predictably took on your alpha - result … blood-splatter on the ceiling.
Same sex animals walk a tight-rope, while opposite sexes tend to relax the rules.  However, when push comes to shove the older will always demand respect, even if it means fisticuffs.

Next  new pup - choose the opposite sex and allow your older dog to instil family values … providing of course it isn’t a crazed, salivating serial killer  … in which case don’t get a puppy!

And if you are presently living in a war zone, remember the younger animal is out of line and should be severely demoted by with holding all privileges in order to restore the natural hierarchy.
Remember your previous packs or think about wild animals especially elephants - wasn’t it always the oldest who called the shots?  So you see, it’s true, size doesn’t count … but age certainly does!